Aluminum plating process

ABSTRACT

A process for depositing a metallic aluminum film on a substrate material which comprises: depositing a transition metal catalyst on said substrate; depositing an aluminum hydride compound on said substrate in contact with said catalyst; contacting said treated substrate in a heated bath consisting of an inert liquid, thereby decomposing said aluminum hydride material and providing a metallic aluminum coating on said substrate; removing said aluminum coated substrate from said bath and removing said inert liquid to provide said metallic aluminum coating on the substrate.

United States Patent Schneggenburger et a1.

[ Feb. 1, 1972 [541 ALUMINUM PLATING PROCESS [72] Inventors: Roger G.Schneggenburger, Freeland; Reinhold Hellmann, Midland, both of Mich.

[731 Assignee: The Dow Chemical Company, Midland,

Mich.

[22] Filed: 0ct.7, 1968 21 Appl. No.: 765,621

[52] U.S.Cl. ..117/37 R, 106/1, 117/47 R, 117/124 C, 117/130 R,117/138.8 R, 117/143 B,

[51] Int. Cl ..C23c 3/04 [58] Field ofSearch ..117/130, 160, 212, 227,107.2, 117/119.8, 46, 37, 47, 124, 138.8, 143; 23/204, 205

[56] References Cited UNITED STATES PATENTS 3,206,326 9/1965 Whaley eta1. ..117/107.2

3,214,288 10/1965 McGraw ..117/130X 3,387,933 6/1968 Snyder ..23/2043,453,089 7/1969 Del Givdice ..23/204 X 3,462,288 10/1969 Schmidt et a1.....117/130 X 2,125,827 8/1938 Turkington... 17/1 19.8 3,256,109 6/1966Berger ..117/212 X Primary Examiner-Ralph S. Kendall Attorney-Griswold &Burdick, William R. Norris and Bruce M. Kanuch 1 ABSTRACT A process fordepositing a metallic aluminum film on a substrate material whichcomprises: depositing a transition metal catalyst on said substrate;depositing an aluminum hydride 10 Claims, No Drawings ALUMINUM PLATINGPROCESS BACKGROUND it is known that metallic aluminum may be plated fromaluminum hydrides by contacting such hydrides with a substrate at orabove the decomposition temperature of the aluminum hydride. Such aprocess usually requires relatively high temperatures to causedecomposition of the aluminum hydride and therefore cannot be used toplate aluminum onto many heat-sensitive substrates. Moreover, necessaryheat is usually supplied to the reaction systems by such means asconvection ovens, heat guns, radiant heat and the like. It is difficultto maintain critical temperature control when employing such heatingmeans. Consequently the plates thus formed are not consistently of auniform adherent nature. It would be highly desirable, therefore, tohave a process which would permit the uniform plating of aluminum atrelatively low temperatures.

Also, since many aluminum hydride compounds are unstable in the presenceof water, it is desirable in order to prepare superior films, todecompose the hydrides in the presence ofa substantially anhydrous inertatmosphere. A further desideratum of the art, therefore, is to provide amethod allowing the use of water-unstable hydrides. It would, therefore,also be desirable to have a process which would permit the plating ofaluminum at relatively low temperatures and to be able to control theatmosphere and temperature within predetermined critical limits. Thiswould allow the plating of heat-sensitive substrates and improve thefilm characteristics of the aluminum plate produced. The present processnow provides a method for preparing aluminum films on various substrateswhile employing lower reaction temperatures and with more precisecontrol over the process conditions.

SUMMARY The present invention comprises depositing on a substrate atransition metal catalyst capable of decomposing aluminum hydrides toform metallic aluminum coatings at a relatively low temperature;depositing on said substrate in the presence of the catalyst an aluminumhydride compound; immersing said catalyst and aluminum hydride coatedsubstrate in a heated inert liquid bath to decompose the aluminumhydride compound thereby forming a metallic aluminum coating on thesubstrate; removing the aluminum coated substrate from the bath andremoving the excess inert liquid.

PREFERRED EMBODIMENT In the practice of the present inventionsubstantially any solid material is suitable as a substrate. Forexample, metals such as iron, magnesium, brass and copper, polymers suchas polyolefins, polyamides and polymeric fluorocarbons, glass, paper,cloth, carbon and graphite, wood, ceramics and the like can all beplated with aluminum by the process of this invention. The nature of thesurface being plated determines to a large extent the brightness of thealuminum plate. In general, the use of a smooth, nonporous surface suchas found on most metals and some polymer films produces a brighter platethan a relatively porous surface such as those encountered with paper orcloth. On the surfaces of some polymers such as polyethylene,polytetrafluoroethylene, acrylonitrile-butadiene-styrene terpolymers andpolypropylene, it has been found that even better adhesion of thealuminum plate is achieved if the surface has been made more polar e.g.,by sulfonation, corona discharge and the like, prior to plating with thealuminum. Various shaped objects may also be plated by the process suchas, for example, whisker materials, reflectors, automobile trim andother decorative objects.

The term aluminum hydride is used herein in its broad sense and is meantto include any hydride compound which contains at least one aluminumatom to which at least one hydrogen atom is directly bonded and includesboth the solvated and nonsolvated forms of those aluminum hydridesoccuring in both forms. Included, therefore, are aluminum trihydride,the substituted aluminum hydrides such as those having the empiricalformula AlH,,X wherein X is a halogen, or -OR group or an lR group(wherein R is an alkyl, substituted alkyl, aryl or substituted arylgroup) and n has a numerical value equal to or less than three. Alsoincluded are the complex aluminum hydrides such as LiAll-h, NaAlH.,,Mg(All-L,) and the like and complex substituted aluminum hydrides suchas those having the empirical formula M(All-lmX., m),, wherein X has thedefinition given above. 111 has a numerical value equal to or less thanfour and M is a metal or mixture of metals, preferably an alkali oralkaline earth metal and a has a numerical value equal to the valance ofM. Of particular utility are the relatively simple aluminum hydridescontaining at least two hydrogen atoms attached to the aluminum, e.g.,All-l AlH Br, LiAll-l and the like. Mixtures of the various aluminumhydrides may also be employed. it is usually desirable for ease ofapplication to employ the aluminum hydride in solvated form. Compoundsknown to solvate or form complexes with the aluminum hydrides includeethers and other oxygen-containing organic compounds, and compoundscontaining a functional group such as a divalent sulfur atom, ortrivalent nitrogen or trivalent phosphorous atom which is capable ofallowing the solvation of an aluminum hydride with such compound. It isusually preferred that the solvate be an etherate and a wide variety ofethers containing from about two to about 20 carbon atoms are suitable.Usually the lower aliphatic ethers such as ethyl, propyl, or butylethers are employed but those containing an aromatic group such asmethylphenyl ether, ethylphenyl ethers, propylphenyl ether or thealicyclic ethers such as tetrahydrofuran and the like may be employed.

In general, to achieve ease and uniformity of application of thealuminum hydride compound, any solvent or mixture of solvents orsuspending agents for the aluminum hydride may be employed which willnot react with the aluminum hydride beyond the formation of a complex orsolvate Suitable solvents include aromatic hydrocarbons such as benzene,toluene and xylene, aliphatic hydrocarbons such as hexane, ethers,tertiary amines and the like.

If desired, such aluminum hydrides may be prepared in situsimultaneously with the plating step by employing aluminumhydride-forming reactants such as mixtures of lithium aluminum hydrideand aluminum chloride, or sodium aluminum hydride and aluminum bromide,or the like. The presence of a metal halide such as LiCl, MgCl or AlCltogether with the aluminum hydride is not detrimental to the platingreaction.

In order to produce decomposition of the aluminum hydrides below theirnormal decomposition temperatures and to cause the aluminum thusproduced to form a coating or plate on the substrate, it is necessary tocontact the aluminum hydride with certain transition metal decompositioncatalysts. Transition metal decomposition catalysts useful herein arecompounds of the metals occurring in Groups WI; and Vb of the periodictable. in instances where the catalyst is applied to the substrate in asolvent, it is preferable that the metal be in the form of a compoundwhich is soluble to the extent of at least 1X10 weight percent of thesolvent employed. For example, such compounds as ZrCl NbCl VOCl VOClTiCI,-2(C H O], TiCl TiBr VCl Ti(OC i-l Cl 2( s 7)2, Z H Z m 4)3'[( 25)2 have proved effective. Some of the transition metal catalystsdefined herein have a more pronounced effect than others in lowering thedecomposition temperature of the aluminum hydride. The chlorides,bromides and oxychlorides of titanium, niobium, vanadium and zirconiumgenerally seem to be more effective than the other compounds of Group Nband Vb transition metals and TiCl, has been found particularly effectivein achieving lower temperature decomposition of the aluminum hydridesand plating of the aluminum thus produced. If the defined catalysts arenot employed, undesirably high temperatures are required to producedecomposition of the aluminum hydride. At such elevated temperatures,even when decomposition is achieved, there is usually no aluminumcoating or plate formed thereby.

The transition metal decomposition catalyst is preferably applied to thesubstrate prior to contact with the aluminum hydride. Such decompositioncatalyst may be applied to the substrate directly as a finely dividedsolid, as a liquid solution or suspension or, where the nature of thecatalyst and the substrate permit, deposited by vapor deposition.Preferably, however, the substrate is contacted with a sufficientquantity of a relatively dilute solution of the catalyst to wet thesurface of the substrate. The solvent for the catalyst is then removed,e.g., by evaporation, leaving the catalyst substantially uniformlydispersed over the surface to be plated. Catalyst solutions at leastabout 1X10 weight percent in decomposition catalyst, and preferably inconcentrations of from about 5X10to about 100 weight percent of catalystwhen applied to the substrate provide sufficient catalyst to achieveplating of aluminum from an aluminum hydride at a significantly lowertemperature than is possible where no catalyst is employed. It has beenfound that uniformity of distribution of the catalyst on the substratehas a significant effect on both the uniformity and thickness of thealuminum plate. It is, therefore, desirable to apply the catalyst to thesubstrate in a manner which will assure relatively uniform distribution.

For substrates, such as magnesium metal and some solid polymers, havingsurface characteristics making uniform distribution of a catalystsolution or aluminum hydride difficult to achieve, it has been foundadvantageous to add to such solution a small amount, e.g., from about0.000] to about 5.0 weight percent, of a wetting agent. Suitable wettingagents include, for example, stearates such as sodium or aluminumstearate or aluminum alkoxides such as aluminum isopropoxide.

Solvents for the transition metal decomposition catalysts are thosenormally liquid materials in which the catalyst is soluble to at leastthe extent of 1X10 weight percent, which will not adversely effect thesubstrate and which will not change the anion of the catalystsufficiently to render it insoluble. Suitable solvents includenonreactive solvents such as benzene, hexane, and halogenatedhydrocarbons, reactive solvents such as alcohols, aldehydes, ketones,mercaptans, carboxylic acids and mineral acids, and coordinatingsolvents such as ethers, nitriles, amides and amines.

By application of the transition metal decomposition catalyst to onlyselected areas of the substrate, it is possible to form an aluminumplate only on such selected areas. In this manner, ornamental designs,outlines, printed circuits and the like may be produced. Likewise, allor a portion of a selected substrate may be coated or plated withaluminum to enhance the ability of such surface to adhere to othermaterials. Of particular utility is the aluminum coating of glass,ceramic, metal or polymer surfaces to enhance their bonding to adhesivepolymers and copolymers such as the copolymers of ethylene and acrylicacid.

Once the desired form and quantity of decomposition catalyst is appliedto the substrate, the catalyzed substrate surface is contacted with asuitable form of aluminum hydride. In

general, it is desirable to apply the aluminum hydride as a solution orsuspension from 0.1 to 1.0 molar or more in aluminum hydride which maybe applied by dipping, spraying or other suitable means. However, goodresults are also achieved by contacting the catalyzed substrate surfacewith a finely divided solid aluminum hydride.

The solvent for the aluminum hydride compound is usually evaporated andthe so-treated substrate is then immersed in the hot inert liquid bath.

The inert liquid bath can consist of any substance which is a liquid atelevated temperatures, usually at temperatures of from 50 to about 200C. and which is substantially inert to aluminum hydride and theparticular substrate employed. The bath should be substantiallyanhydrous, i.e., usually containing less than about parts per million ofwater, due to the sensitivity of most aluminum hydrides to moisture.Likewise, the

'application of the aluminum hydride compound to the sub strate materialshould be conducted in a substantially anhydrous inert atmosphere forbest results. Suitable materials which can be employed for the inertliquid bath consist of, for example, organic substances such as naphtha,ethylbutylbenzene, ethylbenzene, diphenyl ether, octadecane, xylene andother like materials. Other inert aliphatic, aromatic and ethercompounds which are liquid at the indicated tempera ture range can beemployed. The inert liquid bath, however, should not be prepared fromcompounds which contain active species such as alcohols, ketones,aldehydes, esters, acids and unsaturated aliphatics since the aluminumplate which develops will be dark, uneven and the substrate will containunplated areas. It has also been found convenient, although notessential to employ an inert liquid bath which boils in the range ofabout to about C. Such a bath will allow the application of heat forproducing the aluminum plate, and evaporate quickly to provide an evenadherent aluminum plate on the substrate. Higher boiling inert liquidscan be removed, for example, by wiping or draining the coated substratematerial.

As indicated previously the bath should usually be maintained at atemperature of about 50 to about 200 C. A temperature of from about 100to about C. is usually preferred. The actual decomposition temperatureof aluminum hydride compounds catalyzed by the transition metalcatalysts defined herein will vary depending on the particular aluminumhydride compound employed, upon the catalyst used and to some extent,upon the catalyst concentration. Such decomposition temperatures will,however, be substantially lower than those required where no catalyst ispresent.

The present process can be run as a continuous operation for the coatingof various shaped objects such as films, webs, paper and the like. Also,it may be employed to coat various other objects such as whiskermaterials, beads, powders, various irregularly shaped forms ofreflectors for lights, piping and the like. Thus, by employing thetransition metal catalyst and inert liquid bath, aluminum coatings canbe applied to heatsensitive substrates. Likewise, more precise controlover the process conditions can be had by use of the bath with theresult that aluminum coatings of superior quality can be prepared.

The following examples will facilitate a more complete understanding ofthe present invention.

EXAMPLE 1 Several irregularly shaped metal objects were first coatedwith a metal catalyst consisting of TiCL, dissolved in an inert solventconsisting of diethyl ether and the solvent evaporated. Next, an All-lsolution consisting of 0.25 molar AlH in diethyl ether was applied tothe samples and the solvent again evaporated. A substantially water-freebath consisting of naphtha was then prepared and heated to a temperatureof about l30-l40 C. The objects were submerged in the bath for about 5to 10 seconds whereupon a shiny adherent uniform aluminum film wasdeposited onto the objects. The objects were removed from the baths andthe excess naphtha allowed to evaporate from the surface of the aluminumplate.

EXAMPLE 2 In a manner similar to that described in example 1, 2 milthick Mylar brand polyester film samples were treated with the catalystand All-l solution as described in example 1 and then placed in severaldifferent heated baths. All the baths were heated to about 130 C. Thebaths employed consisted of an eutectic mixture of diphenyl and diphenyloxide; ethyl butyl benzene; naphtha and a 200 centistoke methyl endblocked dimethylpolysiloxane. All the samples were plated with a shinyadherent Al film.

Various modifications may be made in the present invention withoutdeparting from the spirit or scope thereof for it is un derstood that weare limited only as defined in the appended claims.

What is claimed is:

ll. In the process of plating metallic aluminum onto a substrate whereincoating a substrate with an aluminum hydride compound in the presence ofan aluminum hydride decom position catalyst comprising a compoundselected from the group consisting of compounds of the metals of GroupsWI: and Vb of the periodic table, and subjecting the coated substrate tosufficient energy to plate said aluminum onto the sub strate, theimprovement which comprises:

a. contacting said substrate with an inert liquid bath, maintained at asufficient temperature to decompose said aluminum hydride compoundthereby depositing metallic aluminum on said substrate;

b. removing the aluminum coated substrate from said bath;

and

c. removing excess inert liquid from said aluminum plated substratematerial.

2. The process as defined in claim 1 wherein said aluminum hydridecompound is a solvated aluminum hydride compound.

3. The process as defined in claim ll wherein said decompositioncatalyst and said aluminum hydride compound are applied to saidsubstrate material as liquid dispersions containing said decompositioncatalysts and said aluminum hydride compounds.

ll. The process as defined in claim 3 wherein said liquid dispersionscontain a surface wetting agent.

5. The process as defined in claim ll wherein said inert bath consistsof a substantially anhydrous material which is a liquid at a temperatureof from about 50 to 200 C., and is further characterized as beingnonreactive with said aluminum hydride compounds, said catalystcompounds and said substrate material.

6. The process as defined in claim 1 wherein said inert bath consists ofa substantially anhydrous material which is a liquid at a temperature offrom about to C. and is further characterized as being nonreactive withsaid aluminum hydride compound said catalyst compound and said substratematerial.

7. The process as defined in claim ll wherein said inert bath ismaintained at a temperature of from about 50 to 200 C.

8. The process as defined in claim ll wherein said decompositioncatalyst is applied to the substrate as a solution containing at least1X10 weight percent catalyst and the aluminum hydride compound isapplied to the catalyst-treated substrate in a liquid dispersioncontaining at least 0.0001 weight percent of said aluminum hydridecompound.

9. The process as defined in claim 11 wherein said substrate is a shapedobject of a material selected from the group consisting of organicpolymers, glasses, ceramics, metals and cellulose fibers.

10. The process as defined in claim 1 wherein said decompositioncatalyst is applied to only a portion of the substrate to therebyproduce a metallic aluminum design.

P0405) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3, 39, 39 Dated 1 February 1972 Inv n Roger G. Schneggenburger andReinhold Hellmann It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2, line 60, add a bracket so it reads: --TiCl -2[(C H O],

line 61, add --2-- so it reads: T1(BH1+)3-2[(C2H5)2O].

Column 3, line 1h, change"5 x 10"" to 5 x 10 Signed and sealed this 8thday of August 1972.

(SEAL) Attesm EDWARD M.FI..ETCHEZR,JH. ROBERT GOTTSCHALK AttestingOfficer Commissioner of Patents

2. The process as defined in claim 1 wherein said aluminum hydridecompound is a solvated aluminum hydride compound.
 3. The process asdefined in claim 1 wherein said decomposition catalyst and said aluminumhydride compound are applied to said substrate material as liquiddispersions containing said decomposition catalysts and said aluminumhydride compounds.
 4. The process as defined in claim 3 wherein saidliquid dispersions contain a surface wetting agent.
 5. The process asdefined in claim 1 wherein said inert bath consists of a substantiallyanhydrous material which is a liquid at a temperature of from about 50*to 200* C., and is further characterized as being nonreactive with saidaluminum hydride compounds, said catalyst compounds and said substratematerial.
 6. The process as defined in claim 1 wherein said inert bathconsists of a substantially anhydrous material which is a liquid at atemperature of from about 100* to 140* C. and is further characterizedas being nonreactive with said aluminum hydride compound said catalystcompound and said substrate material.
 7. The process as defined in claim1 wherein said inert bath is maintained at a temperature of from about50* to 200* C.
 8. The process as defined in claim 1 wherein saiddecomposition catalyst is applied to the substrate as a solutioncontaining at least 1 X 10 6 weight percent catalyst and the aluminumhydride compound is applied to the catalyst-treated substrate in aliquid dispersion containing at least 0.0001 weight percent of saidaluminum hydride compound.
 9. The process as defined in claim 1 whereinsaid substrate is a shaped object of a material selected from the groupconsisting of organic polymers, glasses, ceramics, metals and cellulosefibers.
 10. The process as defined in claim 1 wherein said decompositioncatalyst is applied to only a portion of the substrate to therebyproduce a metallic aluminum design.